1. Field of the Invention
The present invention relates to a plasma display panel and a method of driving it. More particularly, it relates to a plasma display panel adapted to reduce deterioration in operating margin of a plasma display panel device, the deterioration being caused with time by variations with time in characteristics of a protective layer surface of the plasma display panel, and to a method of driving the plasma display panel.
2. Description of the Related Art
Conventionally, plasma display panels as gas discharge display devices have been used for display terminals on the TV, computer or like. Recently, a number of producers and universities have actively conducted research and development on the use of plasma display panels for the information display terminal or the wall-mount TV. With significant progress of information-oriented society, plasma display panel devices, as digital display devices, have been expected to serve as the multi-media monitor as well.
Referring to FIG. 10, the structure of a plasma display panel (hereafter referred to as a PDP) will be explained. FIG. 10 is an exploded view schematically illustrating the structure of a pixel of the PDP in perspective. A front substrate 10 has two display electrodes 11 and 12 arranged substantially in parallel. These display electrodes 11 and 12 are provided in this order in a large number over the entire surface of the front substrate 10. The display electrodes 11 and 12, also referred to as sustain display electrodes, typically comprise transparent electrodes 11i and 12i as well as bus electrodes 11b and 12b formed thereon, respectively. The substrate 10 has also a dielectric layer 13 covering these electrodes 11 and 12, as well as a protective layer 14 formed on the dielectric layer 13. The protective layer 14 is mainly made of MgO. Typically, the thickness of the front substrate 10 is about 2–3 mm, the thickness of the dielectric layer 13 is several tens μm, and the thickness of the protective layer 14 is about 1 μm.
A rear substrate 20, on the other hand, has address electrodes 21 in a direction intersecting the sustain electrodes 11 and 12, and are covered with a dielectric layer 23. Barrier ribs 25 are provided between the address electrodes 21. Phosphor layers 26R (red), 26G (green) and 26B (blue) are each formed between the barrier ribs on the upper surface of the dielectric layer 23 and the side walls of the barrier rib. Shown in FIG. 10 is only one set of phosphor layers 26R, 26G and 26B, though actually provided are a plurality of sets of phosphor layers 26R, 26G and 26B in a number corresponding to the number of pixels of the PDP. Typically, the height of the barrier rib is 100–200 μm.
FIG. 11 shows a block diagram of the construction of a plasma display panel device (hereafter referred to as a PDP device) that includes a circuit for driving the PDP. The sustain electrodes 11 and 12 shown in FIG. 10 are referred to as X and Y electrodes, respectively, which are indicated by Xi (i=1, 2, 3 . . . ) and Yj (j=1, 2, 3 . . . ), respectively, in FIG. 11. The X electrodes are driven by an X sustain circuit 101, and the Y electrodes are driven by a Y scan driver 112 and by a Y sustain circuit 111, in FIG. 11. The address electrodes 21 shown in FIG. 10 are indicated by Ak (k=1, 2, 3 . . . ) and driven by an address driver 121 in FIG. 11.
The lit (ON) or unlit (OFF) state of each cell is selected between the address electrode Ak and the Y electrode Yj. A cell set to the ON state emits light by a sustain discharge for display of a color image. Sustain discharges are carried out between the X electrode and the Y electrode with driving waveforms of voltages applied over the entire display screen.
Now, the driving waveforms and the constitution of a frame will be explained with reference to FIGS. 12(a)–12(c) and 13, respectively.
As shown in FIGS. 12(a), 12(b) and 12(c), the driving waveform is basically comprised of a reset period, an address period and a sustain period. In each period, the waveforms as shown are applied to the X electrode, the Y electrode and the address electrode. Initialization is carried out in the reset period, desired cells are selected in the address period, and sustain discharges for display are generated in the sustain period.
As shown in FIG. 13, each of a plurality of frames constituting one image consists of n sub-frames that correspond to the respective weights of display luminances. Each sub-frame is comprised of the three periods (the reset period, the address period and the sustain period) as shown in FIGS. 12(a), 12(b) and 12(c).
Driving the PDP using both the driving waveforms and the constitution of the frame as shown in FIGS. 12(a)–12(c) and 13 allows performance of gradation display of color image.
To prolong the life of the PDP device with improved stability, it is necessary to reduce deterioration in operating margin of the PDP device, the deterioration being caused with time by “variations with time in performance characteristics”.
Examination on causes of the “variations with time in performance characteristics” has indicated that one cause is “variations in performance characteristics of the address discharge”. The “variations in characteristics of the address discharge” arises from “variations in characteristics (characteristics) of a surface of the protective layer 14” described as follows.
Two kinds of discharge mode within a cell of the PDP give rise to such “variations with time in performance characteristics”. First, these two kinds of discharge mode will be explained with reference to FIG. 14. (In FIG. 14, the X electrode 11 is shown as one electrode by combining together the transparent electrode 11i and the bus electrode 11b, and the same holds for the Y electrode 12 in FIG. 14.)
Of the two kinds of discharge mode, one is a sustain discharge mode indicated by reference numeral 201 in FIG. 14, and the other is an address discharge mode indicated by reference numeral 202.
The sustain discharge 201 is an AC discharge of alternate polarities, which occurs between the X electrode 11 and the Y electrode 12. The sustain discharge 201, as is clear from FIG. 14, is a “surface discharge”, which occurs over a surface of one substrate (the surface of the protective layer 14).
The address discharge 202, on the other hand, is what is known as a kind of DC discharge, typically of a single polarity, which occurs between the address electrode 21 and the Y electrode 12. The address discharge 202 is an “opposite discharge”, which occurs between two substrates.
Next, “variations with time in performance characteristics” relevant to the present invention will be explained.
Ions generated during the sustain discharge 201 collide against the surface of the protective layer 14 disposed on both the X electrode and the Y electrode, thereby gradually sputtering the protective layer 14. Substances thus produced by such sputtering, trace amounts of impurities present in a discharge gas or the like may adhere to the surface of the protective layer 14. Such sputtering of ions, adhesion of impurities or the like are attributed to variations in characteristics of the surface of the protective layer 14 (in characteristics of secondary electron emission yield γ and the like).
Thus, the sustain discharge 201 causes the “variations in performance characteristics of the surface of the protective layer 14”, which in turn vary performance characteristics of the address discharge 202. This is because the address discharge 202 is a DC discharge in which typically, the address electrode 21 serves as the anode and the Y electrode 12 serves as the cathode. Variations in characteristics of a portion of the surface of the protective layer 14 on the cathode (especially, in characteristics of secondary electron emission yield γ and the like) cause variations in characteristics of the address discharge 202.
After a long-term use of the PDP, a voltage in the address discharge 202 may either rise or lower depending on the driving method or the driving waveform of the address discharge 202. In any case, however, the voltage varies with time from an initial voltage. Variations in characteristics (variations in characteristics between cells) that a PDP originally has; difference in frequency of use between cells depending on the manner of the screen display; or the like, increase variations in characteristics of the surface of the protective layer 14 (especially of secondary electron emission yield γ and the like), thereby widening “variations in characteristics (variations in voltage) of the address discharge 202”. “Variations in characteristics (variations in voltage) of the address discharge 202” lead to gradual deterioration in operating margin of the PDP device.
Thus, a “mechanism of deterioration with time” has been clarified as follows: The sustain discharge 201 (the surface discharge) causes gradual and long-term variations in “characteristics of the surface of the protective layer 14”, which lead to “variations in characteristics of the address discharge 202 (opposite discharge)”, with the result of especially deteriorating “operating margin of the address discharge 202”. Deterioration in operating margin finally shortens the life of the PDP device.
Variations in characteristics of the surface of the protective layer 14 cause variations in both the characteristics of the sustain discharge 201 and those of the address discharge 202, though it has been found that ordinarily, the variation ratio is larger in the address discharge 202. To prolong the life of the PDP device, therefore, it is especially important to reduce variations in characteristics of the address discharge 202.